N. Van Wieringen

Delineation of potential hot spots for hyperthermia treatment planning optimisation

The optimal feed parameters of the generators for a complex-phased hyperthermia array system consisting of 4, 8 or even more applicators cannot be found using only the expertise of the treatment staff or using the limited amount of field and temperature data obtained during treatment. A number of strategies have been proposed to help us with the task to optimise the hyperthermia treatment, including several strategies specifically addressing the occurrence of hot spots. Each of the latter strategies strongly relies on the specification of the potential hot spots. This specification is either based on anatomy or the selection of an arbitrary number of potential hot spots. Therefore it is not guaranteed that all potential hot spots are included. This paper introduces a procedure for the delineation and visualisation of potential (SAR) hot spots. The potential hot spots are delineated by selecting those points for which the maximal SAR exceeds a specific SAR selection level. This SAR selection level is defined relative to the highest achievable SAR in the target volume for a certain fixed heating power. A larger number of potential hot spots and hot spots of larger size are delineated if the selection level is decreased. Although the procedure still includes an arbitrary selection criterion, i.e. the selection level, the selection is solely based on calculated EM-field data. As a result all potential hot spots can be delineated a priori. Three different objective functions are applied to maximise the SAR in the target. The first only maximises the SAR in the target volume for a given system power output. The other two intrinsically set a constraint on the set of potential hot spots as a whole. Additionally the SAR in each delineated potential hot spot separately can be constrained. In two patient cases the SAR in potential hot spots can be kept below the selection value applied for delineation of the potential hot spots. If assessed in terms of constraining the SAR value below the selection level while maximising target heating efficiency the combination of an objective function only maximising the SAR in the target with a separate constraint on each potential hot spots appears to be the most efficient.

Modelling tissue heating with ferromagnetic seeds

Interstitial hyperthermia using ferromagnetic seeds demands accurate treatment planning: the seed characteristics and implant geometry must be determined prior to the treatment. A new, finite difference based, seed modelling method is presented. The seed, together with all its surrounding (non-tissue) layers is described as one unit, independent of the tissue grid. The calculation of the seed-tissue interaction is based on the local seed temperature and several tissue temperature samples in the direct vicinity. All the layers between the seed and the surrounding tissue are taken into account in this interaction calculation. The presented implementation describes the analytical solution of the modelled steady-state configurations very accurately. The separation between tissue and seed allows easy assessment of the resulting seed temperature profile which is essential to the optimization of the seed characteristics in treatment planning. The thermal effect due to blood flow in the modelled tissue volume surrounding the seed can be accounted for by inclusion of a heat sink term as well as by inclusion of realistic discrete vasculature.

The effect of catheters and coatings on the performance of palladium-nickel thermoseeds: evaluation and design of implantation techniques.

In the development of materials for self-regulating thermoseeds much effort is put in improvement of the self-regulating temperature control mechanism of the seeds. The catheters and coatings which are needed to implant the seeds or to guarantee biocompatibility, generally impair the optimized performance of the ferromagnetic seeds. The influence of various coatings on the performance of PdNi seeds has been investigated by means of one-dimensional modelling and calorimetric experiments. Implantation using thin walled catheters is acceptable provided that the catheters are filled with water to assure good thermal coupling. Air layers inside catheters should be avoided as they reduce the sharp gradient of the heat production at the Curie temperature significantly. An alternative for the application of catheters is to insert the seeds into metallic needles. The effect of shielding by the metal needle can be minimized by driving the seed into its saturated state using a high magnetic field strength. The thermal interaction between the seed and surrounding tissue can also be enhanced by placing PdNi, e.g. tubular, on the outside of the catheter or brachytherapy needle. An additional advantage of this new design is an increase in the heat production and the quality of temperature control due to an increase in the amount of PdNi. For permanent implantation seeds can be coated with an inert metal, ceramics or plastic. The performance of the seeds is not affected by any of the coatings if certain conditions are met. For plastic coatings the thickness of the coating has to be very thin, preferably < or = 20 microns, to avoid thermal isolation.

Ferromagnetic Techniques for Interstitial Hyperthermia

An interesting hot source technique without the need of external temperature regulation consists in the implantation of tissue by ferromagnetic needles in combination with the use of an externally applied radiofrequency magnetic field. This technique for inducing hyperthermia in tumors has been under investigation since about 1982 at several centers.

OC-0075: Simple spatula improves the geometrical accuracy of a cranial mask for brain tumor radiotherapy

Purpose or Objective The demands on accuracy in radiotherapy have increased especially in stereotactic treatments using IMRT or VMAT techniques where margins to the PTV are as small as 1 or 2 mm, but also for non-stereotactic treatments. Commercial systems providing better geometrical accuracy in patient positioning than conventional standard three-point masks, entail high costs. Although translations can be corrected with a couch shift, rotations cannot be corrected with a standard treatment couch and need to be prevented as much as possible. Therefore we aim to investigate whether the use of a dental fixation created with an inexpensive and simple wooden spatula, will improve accuracy in patient positioning. Material and Methods In 40 patients receiving non-stereotactic cranial radiotherapy, 144 conebeam CTs (CBCT) were acquired prior to treatment. Twenty patients had a standard threepoint thermoplastic mask with a standard base (MacroMedics®, Waddinxveen, The Netherlands); next 20 patients had an identical mask and base, but with the addition of a dental fixation moulded by a wooden spatula, to create an extra point of fixation between the teeth rows. Patients were asked to bite gently on the wooden spatula during moulding of the mask to create an indentation in the mask for dental fixation. After cooling and hardening of the mask, the wooden spatula is removed. During the acquisition of the planning CT and all treatment fractions patients are instructed to bite gently on the indentation. All CBCTs were registered on bony anatomy of the skull. For patients with an online correction protocol, all data were included. For patients with extended NAL correction protocol, only the data of the first ‘NAL’-phase were included. Thereby, the position inaccuracy was calculated on position errors before a position correction was applied. Individual systematic (Ʃ) errors were calculated and analyzed with Levene test. Individual random errors (σ) were calculated and analyzed with the Mann-Whitney test. Results The table summarizes the group setup errors for both fixation systems. Most errors are smaller when using the three-point mask with dental fixation created with a simple wooden spatula compared to the three-point mask alone. Geometrical accuracy shows significant improvement in the systematic and random error for the rotation over the X axis and the random error for rotation over the Y axis. Conclusion Adding a dental fixation point to a standard three-point cranial mask by a simple wooden specula improves geometrical accuracy, particularly by reducing rotational errors. This may be of clinical importance, since rotational errors cannot be corrected by a standard treatment couch. Although the absolute errors are already small for the standard three-point mask, but given the small effort and the low additional costs of a simple wooden spatula, we decided to accept the mask with dental fixation as our standard for non-stereotactic brain tumor radiotherapy.

OC-0461: Does the dosimetric advantage of prone setup persist in small-margin IMRT for gynecological cancer?

Purpose or Objective: In order to reduce dose to the small bowel, some institutions treat patients with gynecological cancer in prone position using a small-bowel displacement device (belly board). This practice is based on dosimetric advantages found in the past for 3DCRT and/or the use of large margins. It is unknown to what extent those advantages are persistent using modern intensity-modulated delivery techniques (e.g. IMRT or VMAT) and adaptive treatment approaches with small CTV-to-PTV margins. The aim of this study is to determine the best patient setup position (prone or supine) in terms of OAR sparing for various CTV-to-PTV margins and modern dose delivery.

EP-1658: The need for quality assurance of the image guidance process in radiotherapy

Purpose/Objective: This work evaluated the inter-observer variability in CBCT registration for prostate patients treated with IMRT (Intensity Modulated Radiation Therapy). We also demonstrated the importance of daily CBCT registration for this treatment. Materials and Methods: In the first study VMAT plans were produced for 2 prostate cancer patients and a voluntary shift of the isocenter was introduced in the anterior/posterior direction. A total of 22 different VMAT plans were evaluated with 13 plans for the first patient and 9 plans for the second. In the second study, 12 technologists each registered 22 CBCT for 22 different prostate patients. Medical doctors have done the same registrations that was considered as the Gold Standard reference. Registration was done using bony anatomy and adjustments made on by the user on evaluation of the rectum and prostate. Shifts in all three axis were documented for a total of 286 CBCT. Two statistical methods were used to analyse the results. The first was the 95 percentile to calculate the minimum threshold under which the users found similar values. The second was the ANOVA test, followed by the Post-Hoc/Bonferroni test. These tests were used to find differences in inter-observer registration variability and determine whether any individual users performed registrations which differed significantly from those of the other users. Results: The dosimetric study showed that a shift of 5 mm in the posterior direction was enough to deliver a higher dose than acceptable to the rectum in both cases.A different threshold was found by shifting anteriorly, ranging from 5 mm for patient B to 12 mm for patient A. On the other hand, the statistical analysis of the registration study showed that using the 95 percentile, threshold values were demonstrated of 2.1, 3.5 and 7.3 mm in the left/right, target/gun and anterior/posterior respectively. The Anova test showed a low p-value in the target/gun axis but using the Post-Hoc analysis there were no significant differences between the technologists and the medical doctors. Conclusions: This study showed the importance of a daily CBCT/CT registration in prostate radiation treatment. The different studies also showed that partial delegation of the prostate registration fto the technologists is feasible under some security thresholds. However, regular training and evaluation should be done by the medical doctors and physicists. EP-1658 The need for quality assurance of the image guidance process in radiotherapy M. Frank, M. Kamphuis, R. De Jong, A. Bel, M. Hulshof, N. Van Wieringen Academic Medical Center (AMC)/ University of Amsterdam, Radiotherapie, Amsterdam, The Netherlands

SU-E-J-216: A Sequence Independent Approach for Quantification of MR Image Deformations From Brachytherapy Applicators

Purpose: MRI is increasingly used as a single imaging modality for brachytherapy treatment planning. The presence of a brachytherapy applicator may cause distortions in the images, especially at higher field strengths. Our aim is to develop a procedure to quantify these distortions theoretically for any MR-sequence and to verify the estimated deformations for clinical sequences. Methods: Image distortions due to perturbation of the B0-field are proportional to the ratio of the induced frequency shift and the read-out bandwidth of the applied sequence. By reconstructing a frequency-shift map from the phase data from a multi-echo sequence, distortions can be calculated for any MR-sequence. Verification of this method for estimating distortions was performed by acquiring images with opposing read-out directions and consequently opposing distortions. The applicator shift can be determined by rigidly matching these images. Clinically, T2W-TSE-images are used for this purpose. For pre-clinical tests, EPI-sequences with narrow read-out bandwidth (19.5–47.5Hz), consequently large distortions, were added to the set of clinical MRsequences. To quantify deformations of the Utrecht Interstitial CT/MR applicator (Elekta Brachytherapy) on a Philips Ingenia 3T MRI, pre-clinical tests were performed in a phantom with the applicator in water, followed by clinical validation. Results: Deformations observed in the narrow bandwidth EPI-images were well predicted using the frequency-shift, the latter giving an overestimation up to 30%/up to 1 voxel. For clinically applied MR-sequences distortions were well below the voxel size. In patient setup distortions determined from the frequency-shift map were at sub-voxel level (<0.7mm). Using T2W-images larger distortions were found (1–2mm). This discrepancy was caused by patient movement between/during acquisition of the T2W-images with opposing read-out directions. Conclusion: Phantom experiments demonstrated the feasibility of a clinical procedure for quantification of MR-image distortions for any MR-sequence. In a clinical set-up the distortions from a Utrecht interstitial CT/MR applicator are sub-voxel level. This work was partially funded by Elekta Brachytherpy

SU‐E‐T‐326: Repeated CT‐Scans in Pulsed Doserate Prostate Brachytherapy: Assessment of Deviations from the Treatment Plan

Purpose: Estimation of deviations between planned and delivered dose in Pulsed Doserate (PDR) brachytherapy for prostate cancer.Methods: A boost of 28.8 Gy is given with PDR brachytherapy in addition to 46 Gy delivered with External Beam RT. Brachytherapy is given in 24 pulses of 120 cGy, with an interpulse period time of 2.0 hours, resulting in a treatment time of over 46 hours. For 31 patients, additional CT‐scans were made apart from the Treatment PlanningCT, i.e., one at 24 hours after start of PDR treatment and one shortly before finishing PDR treatment. On the second and third CT, the brachytherapy catheters were newly reconstructed and the treating physician delineated the PTV and organs at risk. Dwell positions and dwell times as used for the original Treatment Plan were imported into the newly reconstructed catheters and the dose distribution was recalculated. Plan comparison parameters were prostate V100 and D90 and rectum and bladder D2cc. Results: Averaged over 3 CT scans and all patients, the prostate V100 decreased 1.2% and D90 decreased 2.7%. For rectum, D2cc was within the tolerance dose (96 cGy/pulse) for all patients on the planning CT, but exceeded the tolerance dose on scan #2 in 7/31 patients with maximally 46% and in scan #3 in 5/25 patients with maximally 29%. Also for bladder D2cc was within the tolerance dose (120 cGy/pulse) for all patients on the planning CT. Here, the tolerance dose was exceeded on scan #2 in 2/31 patients with maximally 14% and in scan #3 in 3/25 patients with maximally 28%. Conclusions: In PDR prostate brachytherapy the relatively long treatment time has no clinically relevant deteriorating effect on the dosimetric quality of the treatment. PTV dose hardly deviates from planned dose, while OAR tolerance doses are rarely exceeded and only in small volumes.

SU‐E‐T‐569: Potential Benefit of Dose‐Guided Radiotherapy with On‐Line Dose Evaluation for SBRT for Lung Tumors

Purpose: Stereotactic body radiotherapy(SBRT) is associated with a high biological dose delivered in a small number of fractions. Before treatment the position of the tumor is verified on cone‐beam CT(CBCT) and on‐line position correction is applied. A correction is always verified by a second CBCT. As only translational errors in the tumor position can be corrected, it is hard to determine the effect of anatomical changes of the patient or dose changes in the organs at risk (OARs). The goal of this study was to evaluate the actual dose distribution in stereotactic lungcancer patients to determine whether on‐line dose evaluation could improve the decision making. Methods: The actual dose distributions were determined by recalculating the original treatment plans on CBCT. 108 CBCTs of 10 patients were analyzed retrospectively (54 before and 54 after correction). The actual dose distribution was compared to the original plan. We analyzed the relative change in volume of the internal target volume (ITV) receiving the prescribed dose, dV100%, where a negative value indicates an underdosage with respect to the plan, and we evaluated the volume that exceeded the constraints of the OARs.Results: Before applying position correction dV100% was in the range −2% − 0 in 69% of the cases. In 7% the deterioration before correction was more than 30% with respect to the original plan. The average dV100% was −6.4% (range −75.9 to 0) before correction. After correction the largest error with respect to the plan was − 0.07%. The OARs never exceeded their constraints, neither before nor after correction. Conclusions: Compared to image‐guidedradiotherapy(IGRT),dose‐guidedradiotherapy (DGRT) may prevent the execution of unnecessary position correction and with that, treatment time can be reduced. Moreover, DGRT enables verification of the dose in the ITV and OARs in case of anatomical changes. This work was partially supported by a grant from Elekta.